This invention relates to a photographic polyester multilayer film base having improved properties. The multilayer film base comprises PET-based polyester materials with a specified relationship between the level of monomeric units derived from 1,4-cyclohexane dimethanol (CHDM) in a first layer and the level of 1,4-cyclohexanedimethanol in the other layers of the multilayer film base such that a specified cutting-related property is obtained.
Silver-halide photographic elements comprise one or more light-sensitive layers coated on a support. Typically the support comprises a sheet of a transparent or translucent film, commonly referred to as a film base. Other layers, such as backing or subbing layers, may be laminated onto either side of the film base. Common film-base materials for photographic elements are cellulose triacetate (CTA) and poly(ethylene terephthalate) (PET). More recently it has been proposed to use poly(ethylene naphthalate) (PEN) as a film base for photographic elements which are intended to be used in a cartridge of reduced diameter which requires rolling the film more tightly than previously.
CTA has generally a good mix of physical properties for various types of photographic films. However, its manufacturing process involves high levels of gaseous emissions, and it is relatively costly. The manufacturing process for PET, on the other hand, is environmentally benign Poly(ethylene terephthalate) (PET) films exhibit excellent properties for use as photographic film base with regard to transparency, dimensional stability, mechanical strength, resistance to thermal deformation. However, compared to CTA, PET films are extremely tough and, therefore, not well suited for finishing operations, i.e., slitting, chopping and/or perforating processes, which are required in the manufacture or preparation of photographic films. Moreover, such films are difficult to cut in various steps of the photofinishing process such as splicing, notching, and sleeving. This is one of the reasons that PET materials have been considered unusable as a film base in certain consumer photographic film applications, such as 35 mm film, especially consumer films requiring non-centralized external processing or minilab processing where finishing must be easily handled. PET materials are presently used in photographic films in which less decentralized processing is not required, for example, X-ray films, motion picture films, and graphic arts films. With respect to the latter types of films, adjustments to processing can be more easily made to handle cutting and the like. The process for making a polyester-based photographic film base typically comprises the steps of casting a molten polyester resin in a machine direction onto a casting surface to form a continuous sheet, drafting the sheet by stretching in the machine direction, tendering the sheet by stretching in the transverse direction, heat-setting the drafted and tentered sheet, and cooling the heat-set sheet to form a stretched, heat-set film, such as described in, e.g., U.S. Pat. No. 4,141,735, the disclosure of which is incorporated by reference herein. U.S. Pat. Nos. 5,385,704 and 5,607,826 disclose a method for improving the finishing characteristics of photographic materials employing a PET film base involving lowering the planar birefringence of the film base to below 0.150 by performing a detentering step which allows the tentered film to shrink in width by 2 to 20% (pref. 10-18%) after the heat-setting step during film manufacturing. Improvement in finishing characteristics of PET-based photographic film, as manifested by decrease in dirt and debris generated during finishing operations, is also disclosed in U.S. Pat. No. 6,228,569 and U.S. Ser. No. 09/223,876 hereby incorporated by reference in their entirety. These latter inventions disclose a method utilizing relatively high heat-set temperatures ( greater than 220xc2x0 C.) applied during the film manufacturing process, which substantially improves the finishing and cutting characteristics of PET-based photographic supports. However, even with the demonstrated improvements in finishability, the PET-based film is still difficult to cut in various steps of the photofinishing process.
Another general problem with PET film is its tendency to take up high levels of curl during storage in cartridges at high temperatures and its inability to sufficiently lower this curl during photoprocessing as commonly exhibited by CTA-based photographic films. A solution to the latter problem was proposed in U.S. Pat. No. 5,556,739 to Nakanishi et al, U.S. Pat. No. 5,387,501 to Yajima et al., and U.S. Pat. No. 5,288,601 to Greener et al. in which multilayered supports comprise polyesters modified by sulfonate and other hydrophilic moieties that facilitate, in wet processing, recovery of curl imposed on the film during storage in a cartridge. Another general approach to lowering the tendency of a polyester film base to take up curl (core-set) during storage is through annealing at elevated temperature and/or by raising the glass transition temperature (Tg) of the polyester.
U.S. Pat. No. 3,326,689 to Murayama discloses glow discharge treatment for improved curl of a film base made from a polyester material, preferably a PEN material. In one case, the polyester material comprises a PET-type material in which 25 mol % of the glycol component repeat units are derived from CHDM. U.S. Pat. No. 5,294, 473 to Kawamoto similarly discloses a PET polyester film base in which 25 mol % of the glycol component repeat units are derived from CHDM, with improved (reduced) curl.
U.S. Pat. No. 5,925,507 to Massa et al. discloses a PET film-base material having less tendency to core set, comprising polyester containing at least 30 weight % 1,4-cyclohexane dimethanol (CHDM), which polyester is blended with a polycarbonate that contains bisphenol. U.S. Pat. No. 4,141,735 to Schrader et al. discloses a polyester film base having improved core-set curl, involving the use of heat tempering, in one example using poly(1,4-cyclohexylene dimethylene terephthalate). However, this polymer crystallizes rapidly, therefore the making of its oriented film is difficult. Also, the polymer becomes opaque or hazy and useless for photographic applications where transparency is required.
The use of high heat-set temperature during the film-base manufacturing process has also been used to improve the finishability of PET-based photographic film. However, even with the demonstrated improvements in finishability, the PET-based film is still difficult to cut in various steps of the photofinishing process. U.S. Pat. No. 5,034,263 to Maier et al. disclosed a laminated film comprising a poly(ethylene terephthalate) core and, on at least one surface thereof, an overcoat of a poly(1,4-cyclohexylene dimethylene terephthalate) polyester, in order to allow the laminated film to be readily slit and perforated using techniques commonly employed with consumer film. Maier et al. states that the CHDM component should comprise at least 70 mol % of the glycol component of the polyester. However, such laminates have been found prone to delamination.
The blending or copolymerizing of conventional polyester with other polyester constituents (polymers or comonomers), in order to improve the cutting performance of a film, has also been proposed for PEN-based polyester films, as disclosed in U.S. Pat. No. 6,232,054 B1 to Okutu et al. However, PEN is generally considerably more costly and more difficult to manufacture than PET, so a clear need exists for improving the cuttability of PET-based polyester supports.
Outside the photographic field, poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) are common commercial semicrystalline polyesters, which are widely used for packaging applications due to the combination of desirable properties that they possess. The high oxygen barrier properties of these polyesters render them particularly valuable for packaging oxygen-sensitive food and other goods and materials. PEN has advantages over PET due to its higher Tg and higher oxygen barrier properties, although PEN, as mentioned above, is considerably more costly and is somewhat harder to process than PET.
The toughness and cutting difficulty of PET and similar polyesters is generally attributed to the crystal structure and molecular orientation of the film. It is known that changes in these factors, driven either by formulary changes or by modified process conditions, can be used to lower the toughness and improve the cutting performance of PET. Generally, the crystallinity of PET can be lowered or altogether eliminated by adding suitable crystallization modifiers. Crystallization modifiers like isophthalic acid (IPA) and 1,4-cyclohexane dimethanol (CHDM) are often copolymerized into PET and PEN polyesters to form copolyesters that have better processing properties. Modest levels of IPA slow down crystallization and raise the oxygen barrier properties. Higher levels of IPA break up crystallinity and lead to amorphous copolyesters with good barrier properties, but these copolyesters, are known to those skilled in the art, to possess poor impact and other mechanical properties. Modest levels of CHDM also slow down crystallization, but decrease oxygen barrier properties. Higher levels of CHDM are well known to form families of amorphous copolyesters, which are widely used in commerce in a multitude of applications including heavy gauge sheet, signage, medical packages, etc. These copolyesters have excellent impact resistance and other mechanical properties, but have lower oxygen barrier properties than IPA-modified copolyesters and lower oxygen barrier properties than PET.
Amorphous copolyesters are generally defined as copolyesters that do not show a substantial melting point by differential scanning calorimetry. These copolyesters are typically based on terephthalic acid, isophthalic acid, ethylene glycol, neopentyl glycol and 1,4-cyclobexane dimethanol. It is known that amorphous copolyesters possess a combination of desirable properties, such as excellent clarity and color, toughness, chemical resistance and ease of processing. Accordingly, such copolyesters are known to be useful for the manufacture of extruded sheets, packaging materials, and parts for medical devices. For example. U.S. Pat. Nos. 5,385,773 and 5,340,907 to Yau et al. discloses polyesters of 1,4-cyclohexane dimethanol, in which the diol is present in an amount of 10-95 mol % of the glycol component, and a process for producing such copolymers by esterification. U.S. Pat. No. 6,183,848 B1 to Turner et al. disclose an amorphous copolyester comprising various amounts of comonomers derived from 1,4-cyclohexane dimethanol which, because of improved gas barrier properties, are useful for packaging perishable goods. In one embodiment, the copolyester is disclosed as a biaxially oriented sheet. Film and sheet made from various amorphous PET polyesters comprising repeat units from CHDM are sold by Eastman Chemical Company under the trademark EASTAPAK and EASTAR copolyesters.
A variety of patents disclose a multilayer film base. For example, U.S. Pat. No. 5,034,263 to Maier discloses a multilayer film base comprising a core layer of PET. U.S. Pat. No. 5,387,501 to Yajima discloses a multilayer film base comprising PET in one layer and a second layer in which CHDM may be present. U.S. Pat. No. 5,759,756 to Laney discloses a film base comprising a CHDM-containing core. WO 01/34391 to Moskala, U.S. Pat. No. 5,288,601 to Greener and U.S. Pat. No. 5,556,739 are further examples of multilayer film bases that may contain CHDM.
Accordingly, it would be desirable to provide a PET film base with improved physical properties. In particular, it would be desirable to obtain a PET film base that is less tough and better suited for finishing operations, i.e., slitting, chopping and perforating processes, which are required in the preparation of photographic films. Moreover, it would be desirable to obtain a PET film base that is easier to cut in various steps of the photofinishing process, such as splicing, notching, and sleeving. Additionally, it would be desirable to be able to use PET as a film base in certain consumer photographic film applications and in films processed in a minilab setting. It would also be desirable for such a PET film base. to have other advantageous properties such as dimensional stability and a reduced tendency to take up high levels of cur during storage in cartridges at high temperatures and/or a higher propensity to lower this curl during photoprocessing.
This invention relates to a poly(ethylene terephthalate) photographic film base comprising a multilayer structure, in which a controlled amount of monomeric units derived from 1,4-cyclohexanedimethanol (CHDM) is present in at least two layer, such that the multilayer film base has a specified cutting-related property. This can be accomplished either by the addition/blending of polyester polymers containing CHDM monomeric units to PET material and/or the incorporation of CHDM monomer units into a PET-polymer backbone at appropriate levels.
A further embodiment of the invention is directed towards a photographic element comprising at least one light sensitive silver halide-containing emulsion layer and a PET multilayer film base produced in accordance with the above embodiments.
The multilayer film base of the present invention has desirable properties for use in photographic elements These include good stiffness, low tear strength and improved cuttability. Furthermore, a multilayer film base for photographic film or other elements provides excellent dimensional stability, optical clarity and mechanical strength in addition to possessing an improved cuttability.
In the present invention, monomeric units derived from 1,4-cyclobexane dimethanol (CHDM) are also referred to as xe2x80x9cCHDM repeat unitsxe2x80x9d or xe2x80x9cCHDM-comonomer units.xe2x80x9d Other definitions of terms, as used herein, include the following:
The term xe2x80x9cterephthalic acid,xe2x80x9d as used herein is meant to include suitable synthetic equivalents such as dimethyl terephthalate. It should be understood that xe2x80x9cdicarboxylic acidsxe2x80x9d includes the corresponding acid anhydrides, esters and acid chlorides for these acids. Regarding the glycol/diol component or acid component in the polyester material, the mole percentages referred to herein equal a total of 100 mol % each.
xe2x80x9cPET polymer,xe2x80x9d xe2x80x9cPET resin,xe2x80x9d xe2x80x9cpoly(ethylene terephthalate) resin,xe2x80x9d and the like refers to a polyester comprising at least 98 mol % terephthalic-acid comonomer units, based on the total acid component, and comprising at least 98 mol % of ethylene-glycol comonomer units, based on the total glycol component This includes PET resins consisting essentially of about 100 mol % terephthalic-acid comonomer units, based on the total acid component, and consisting essentially of about 100 mol % of ethylene-glycol comonomer units, based on the total glycol component.
The term xe2x80x9cmodified PET polymer,xe2x80x9d xe2x80x9cmodified PET resin,xe2x80x9d or the like is a polyester comprising at least 70 mol % terephthalic-acid comonomer units, based on the total acid component, that has been modified so that either the acid component is less than 98 mol % of terephthalic-acid (xe2x80x9cTAxe2x80x9d) comonomer units or the glycol component is less than 98 mol % of ethylene-glycol (xe2x80x9cEGxe2x80x9d) comonomer units, or both the TA and EG comonomer units are in an amount less than 98 mol %. The modified PET polymer is modified with, or copolymerized with, one or more comonomers other than terephthalic-acid comonomers and/or ethylene-glycol comonomers in an amount of greater than 2 mol % (including greater than 5 mol %), of either the acid component and/or the glycol component, for example, to improve the cuttability of a film base or otherwise change the properties of the filmbase in which it is used. The xe2x80x9cmodified PET resinxe2x80x9d does not necessarily need to contain any ethylene-glycol comonomer units, and it does not necessarily need to contain any acid component other than terephthalic-acid comonomer units.
In one preferred embodiment, the xe2x80x9cmodified PET polymerxe2x80x9d is a polyester comprising at least 80 mol % terephthalic-acid comonomer units, based on the total acid component, and at least 60 mol % ethylene glycol (EG) comonomer units, further modified with or copolymerized with one or more additional types of comonomers, preferably in the amount of greater than 5 mol % of the acid component and/or glycol component.
The tern xe2x80x9cCHDM-modified PETxe2x80x9d or xe2x80x9cCHDM-modified-PET polyesterxe2x80x9d refers to a modified-PET polymer modified by the inclusion of at least 2 mol % (including at least 3.5 mol %) CHDM comonomer units. In one preferred embodiment, a modified-PET polymer is modified by the inclusion of at least 65 mol % CHDM-comonomer units, based on the total glycol component.
Similarly, the term xe2x80x9cCHDM-modified polyesterxe2x80x9d refers to a polyester comprising at least 2 mol % (including at least 3.5 mol %) CHDM-comonomer units, based on total glycol component, but not necessarily comprising any specific amount of terephthalic acid component in one embodiment, a CHDM-modified polyester comprises at least 65 mol % CHDM-comonomer units, based on total glycol component, but not necessarily comprising any specific amount of terephthalic-acid comonomer units.
The term xe2x80x9chigh-CHDM-modified PETxe2x80x9d refers to a CHDM-modified PET polyester in which the level of CHDM-comonomer units is equal to or greater than 95 mol % (including 100 mol %). This includes both xe2x80x9cPCTxe2x80x9d (polycyclohexylene dimethylene terephthalate) and xe2x80x9cPCTA,xe2x80x9d which is a copolymer of three monomers: terephthalic acid, isophthalic acid and 1,4-cyclohexane dimethanol, with 100 mol % of the 1,4cyclohexane dimethanol based on its glycol component.
The term xe2x80x9chigh-CHDM-modified polyesterxe2x80x9d refers to a CHDM-modified polyester in which the level of CHDM-comonomer units is greater than 95 mol % (including 100 mol %), but not necessarily comprising any amount of terephthalic-acid comonomer units.
xe2x80x9cPET-based polyester materialxe2x80x9d is a semicrystalline material comprising one or more polymers wherein at least 70 % by weight of the material is one or more polymers that are either a PET polymer or modified PET polymer. Optionally, the material may also include addenda such as silica beads, plasticizers and the like.
A film base is made using a PET-based polyester material in the present invention. In one embodiment, preferably greater than 80 % by weight, more preferably greater than 90 % by weight, of the PET-based polyester material used in this invention is one or more polymers that are either a PET polymer or modified PET polymer.